I ride motorcycles almost exclusively in the game and the Mac 10 is the best choice for blowing up Freddy's PCJ. I alternate between the MP-5 because of it's range while taking over gang territories, but the Mac 10 shines as a drive by weapon in this instance so I stock up.

since AmmuNation isn't open (usually) at this point in the game I get what I can by spraying over all the 100 tags and stocking up on the found weapons underneath the bridge behind CJ and Sweet's houses and the Tec 9 on top of Sweet's house

The key to this mission is to simply KEEP UP with him and stay on his trail. If he gets too far ahead, you fail the mission. Considering that he goes through just about every single alley in Los Santos, your best strategy is to drive a little slower, using the handbrake for turns. Every time you fall off the bike, he gets farther ahead of you. So you will need to keep up with him, which is best accomplished by NOT driving at breakneck speeds, but instead, driving just fast enough to keep up with him. You can wait until he gets off his bike to kill him, which is a lot easier. I do believe that you can shoot him off the bike, but that becomes a little problematic if you aren't a great bike rider.